Tissue-specific activation of the osmotin gene by ABA, C2H4 and NaCl involves the same promoter region

OsOLP1 contributes to drought tolerance in rice by regulating ABA biosynthesis and lignin accumulation

Rice, as a major staple crop, employs multiple strategies to enhance drought tolerance and subsequently increase yield. Osmotin-like proteins have been shown to promote plant resistance to biotic and abiotic stress. However, the drought resistance mechanism of osmotin-like proteins in rice remains unclear. This study identified a novel osmotin-like protein, OsOLP1, that conforms to the structure and characteristics of the osmotin family and is induced by drought and NaCl stress. CRISPR/Cas9-mediated gene editing and overexpression lines were used to investigate the impact of OsOLP1 on drought tolerance in rice. Compared to wild-type plants, transgenic rice plants overexpressing OsOLP1 showed high drought tolerance with leaf water content of up to 65%, and a survival rate of 53.1% by regulating 96% stomatal closure and more than 2.5-fold proline content promotion through the accumulation of 1.5-fold endogenous ABA, and enhancing about 50% lignin synthesis. However, OsOLP1 knockout lines showed severely reduced ABA content, decreased lignin deposition, and weakened drought tolerance. In conclusion, the finding confirmed that OsOLP1 drought-stress modulation relies on ABA accumulation, stomatal regulation, proline, and lignin accumulation. These results provide new insights into our perspective on rice drought tolerance.

Genome-wide comprehensive characterization and expression analysis of TLP gene family revealed its responses to hormonal and abiotic stresses in watermelon (Citrullus lanatus)

Thaumatin-like protein (TLP) is the well-known sweetest protein which plays a crucial role in diverse developmental processes and different stress conditions in plants, fungi and animals. The TLP gene family is extensively studied in different plant species including crop plants. Watermelon (Citrullus lanatus) is an important cucurbit crop cultivated worldwide; however, the comprehensive information about the TLP gene family is not available in watermelon. In the present study, we identified the 29 TLP genes as gene family members in watermelon using various computational methods to understand its role in different developmental processes and stress conditions. ClaTLP gene family members were not uniformly distributed on 22 chromosomes. Phylogenetic analysis revealed that the ClaTLP gene family members were grouped into 10 sub-groups. Further, gene duplication analysis showed thirteen gene duplication events which included one tandem and twelve segmental duplications. Amino acid sequence alignment has shown that ClaTLP proteins shared 16 conserved cysteine residues in their THN domain. Furthermore, cis-acting regulatory elements analysis also displayed that ClaTLP gene family members contain diverse phytohormone, various defense, and stress-responsive elements in their promoter region. The expression profile of the ClaTLP gene family revealed the differential expression of gene family members in different tissues and abiotic stresses conditions. Moreover, the expression profile of ClaTLP genes was further validated by semi-quantitative reverse transcriptase PCR. Taken together, these results indicate that ClaTLP genes might play an important role in developmental processes and diverse stress conditions. Therefore, the outcome of this study brings forth the valuable information for further interpret the precise role of ClaTLP gene family members in watermelon.

Genome-Wide Identification of the Thaumatin-like Protein Family Genes in Gossypium barbadense and Analysis of Their Responses to Verticillium dahliae Infection

(1) Background: Plants respond to pathogen challenge by activating a defense system involving pathogenesis-related (PR) proteins. The PR-5 family includes thaumatin, thaumatin-like proteins (TLPs), and other related proteins. TLPs play an important role in response to biotic and abiotic stresses. Many TLP-encoding genes have been identified and functionally characterized in the model plant species. (2) Results: We identified a total of 90 TLP genes in the G. barbadense genome. They were phylogenetically classified into 10 subfamilies and distributed across 19 chromosomes and nine scaffolds. The genes were characterized by examining their exon-intron structures, promoter cis-elements, conserved domains, synteny and collinearity, gene family evolution, and gene duplications. Several TLP genes were predicted to be targets of miRNAs. Investigation of expression changes of 21 GbTLPs in a G. barbadense cultivar (Hai7124) resistance to Verticillium dahliae revealed 13 GbTLPs being upregulated in response to V. dahliae infection, suggesting a potential role of these GbTLP genes in disease response. (3) Conclusions: The results of this study allow insight into the GbTLP gene family, identify GbTLP genes responsive to V. dahliae infection, and provide candidate genes for future studies of their roles in disease resistance.

The Osmotin-Like Protein Gene PdOLP1 Is Involved in Secondary Cell Wall Biosynthesis during Wood Formation in Poplar

Osmotin-like proteins (OLPs) mediate defenses against abiotic and biotic stresses and fungal pathogens in plants. However, no OLPs have been functionally elucidated in poplar. Here, we report an osmotin-like protein designated PdOLP1 from Populus deltoides (Marsh.). Expression analysis showed that PdOLP1 transcripts were mainly present in immature xylem and immature phloem during vascular tissue development in P. deltoides. We conducted phenotypic, anatomical, and molecular analyses of PdOLP1-overexpressing lines and the PdOLP1-downregulated hybrid poplar 84K (Populus alba × Populus glandulosa) (Hybrid poplar 84K PagOLP1, PagOLP2, PagOLP3 and PagOLP4 are highly homologous to PdOLP1, and are downregulated in PdOLP1-downregulated hybrid poplar 84K). The overexpression of PdOLP1 led to a reduction in the radial width and cell layer number in the xylem and phloem zones, in expression of genes involved in lignin biosynthesis, and in the fibers and vessels of xylem cell walls in the overexpressing lines. Additionally, the xylem vessels and fibers of PdOLP1-downregulated poplar exhibited increased secondary cell wall thickness. Elevated expression of secondary wall biosynthetic genes was accompanied by increases in lignin content, dry weight biomass, and carbon storage in PdOLP1-downregulated lines. A PdOLP1 coexpression network was constructed and showed that PdOLP1 was coexpressed with a large number of genes involved in secondary cell wall biosynthesis and wood development in poplar. Moreover, based on transcriptional activation assays, PtobZIP5 and PtobHLH7 activated the PdOLP1 promoter, whereas PtoBLH8 and PtoWRKY40 repressed it. A yeast one-hybrid (Y1H) assay confirmed interaction of PtoBLH8, PtoMYB3, and PtoWRKY40 with the PdOLP1 promoter in vivo. Together, our results suggest that PdOLP1 is a negative regulator of secondary wall biosynthesis and may be valuable for manipulating secondary cell wall deposition to improve carbon fixation efficiency in tree species.

A Development-Associated Decrease in Osmotic Potential Contributes to Fruit Ripening Initiation in Strawberry (Fragaria ananassa)

Fruit development and ripening are accompanied by a large increase in cellular soluble solid contents, which results in a significant decrease in osmotic potential (DOP). Here, we report that this development-associated DOP contributes to the initiation of ripening in strawberry (Fragaria ananassa Duch., Benihoppe) fruit. We show that fruit water potential significantly decreases at the onset of ripening as a result of the DOP. Further analysis using nuclear magnetic resonance spectroscopy (NMR) indicated that the change in fruit water potential was likely caused by catabolism of large molecules in receptacle cells, and bioinformatic analysis identified a family of osmotin-like proteins (OLP) that have a potential role in osmolyte accommodation. The gene expression of more than half of the OLP members increased substantially at the onset of fruit ripening, and specifically responded to DOP treatment, consistent with a close relationship between DOP and fruit ripening. We report that the DOP induced either by mannitol or water loss, triggered fruit ripening, as indicated by the elevated expression of multiple ripening genes and diverse ripening-associated physiological parameters. Collectively, these results suggest that the DOP contributes to strawberry fruit ripening initiation.

Transcriptome analysis during ripening of table grape berry cv. Thompson Seedless

Ripening is one of the key processes associated with the development of major organoleptic characteristics of the fruit. This process has been extensively characterized in climacteric fruit, in contrast with non-climacteric fruit such as grape, where the process is less understood. With the aim of studying changes in gene expression during ripening of non-climacteric fruit, an Illumina based RNA-Seq transcriptome analysis was performed on four developmental stages, between veraison and harvest, on table grapes berries cv Thompson Seedless. Functional analysis showed a transcriptional increase in genes related with degradation processes of chlorophyll, lipids, macromolecules recycling and nucleosomes organization; accompanied by a decrease in genes related with chloroplasts integrity and amino acid synthesis pathways. It was possible to identify several processes described during leaf senescence, particularly close to harvest. Before this point, the results suggest a high transcriptional activity associated with the regulation of gene expression, cytoskeletal organization and cell wall metabolism, which can be related to growth of berries and firmness loss characteristic to this stage of development. This high metabolic activity could be associated with an increase in the transcription of genes related with glycolysis and respiration, unexpected for a non-climacteric fruit ripening.

Phosphorylation of the transcriptional repressor MYB15 by mitogen-activated protein kinase 6 is required for freezing tolerance in Arabidopsis

The expression of CBF (C-repeat-binding factor) genes is required for freezing tolerance in Arabidopsis thaliana. CBFs are positively regulated by INDUCER OF CBF EXPRESSION1 (ICE1) and negatively regulated by MYB15. These transcription factors directly interact with specific elements in the CBF promoters. Mitogen-activated protein kinase (MAPK/MPK) cascades function upstream to regulate CBFs. However, the mechanism by which MPKs control CBF expression during cold stress signaling remains unknown. This study showed that the activity of MYB15, a transcriptional repressor of cold signaling, is regulated by MPK6-mediated phosphorylation. MYB15 specifically interacts with MPK6, and MPK6 phosphorylates MYB15 on Ser168. MPK6-induced phosphorylation reduced the affinity of MYB15 binding to the CBF3 promoter and mutation of its phosphorylation site (MYB15S168A) enhanced the transcriptional repression of CBF3 by MYB15. Furthermore, transgenic plants overexpressing MYB15S168A showed significantly reduced CBF transcript levels in response to cold stress, compared with plants overexpressing MYB15. The MYB15S168A-overexpressing plants were also more sensitive to freezing than MYB15-overexpressing plants. These results suggest that MPK6-mediated regulation of MYB15 plays an important role in cold stress signaling in Arabidopsis.

Phosphorus and magnesium interactively modulate the elongation and directional growth of primary roots in Arabidopsis thaliana (L.) Heynh

A balanced supply of essential nutrients is an important factor influencing root architecture in many plants, yet data related to the interactive effects of two nutrients on root growth are limited. Here, we investigated the interactive effect between phosphorus (P) and magnesium (Mg) on root growth of Arabidopsis grown in pH-buffered agar medium at different P and Mg levels. The results showed that elongation and deviation of primary roots were directly correlated with the amount of P added to the medium but could be modified by the Mg level, which was related to the root meristem activity and stem-cell division. High P enhanced while low P decreased the tip-focused fluorescence signal of auxin biosynthesis, transport, and redistribution during elongation of primary roots; these effects were greater under low Mg than under high Mg. The altered root growth in response to P and Mg supply was correlated with AUX1, PIN2, and PIN3 mRNA abundance and expression and the accumulation of the protein. Application of either auxin influx inhibitor or efflux inhibitor inhibited the elongation and increased the deviation angle of primary roots, and decreased auxin level in root tips. Furthermore, the auxin-transport mutants aux1-22 and eir1-1 displayed reduced root growth and increased the deviation angle. Our data suggest a profound effect of the combined supply of P and Mg on the development of root morphology in Arabidopsis through auxin signals that modulate the elongation and directional growth of primary root and the expression of root differentiation and development genes.

Osmotin: a plant sentinel and a possible agonist of mammalian adiponectin

Osmotin is a stress responsive antifungal protein belonging to the pathogenesis-related (PR)-5 family that confers tolerance to both biotic and abiotic stresses in plants. Protective efforts of osmotin in plants range from high temperature to cold and salt to drought. It lyses the plasma membrane of the pathogens. It is widely distributed in fruits and vegetables. It is a differentially expressed and developmentally regulated protein that protects the cells from osmotic stress and invading pathogens as well, by structural or metabolic alterations. During stress conditions, osmotin helps in the accumulation of the osmolyte proline, which quenches reactive oxygen species and free radicals. Osmotin expression results in the accumulation of storage reserves and increases the shelf-life of fruits. It binds to a seven-transmembrane-domain receptor-like protein and induces programmed cell death in Saccharomyces cerevisiae through RAS2/cAMP signaling pathway. Adiponectin, produced in adipose tissues of mammals, is an insulin-sensitizing hormone. Strangely, osmotin acts like the mammalian hormone adiponectin in various in vitro and in vivo models. Adiponectin and osmotin, the two receptor binding proteins do not share sequence similarity at the amino acid level, but interestingly they have a similar structural and functional properties. In experimental mice, adiponectin inhibits endothelial cell proliferation and migration, primary tumor growth, and reduces atherosclerosis. This retrospective work examines the vital role of osmotin in plant defense and as a potential targeted therapeutic drug for humans.

Overexpression of tobacco osmotin (Tbosm) in soybean conferred resistance to salinity stress and fungal infections

Salinity and fungal diseases are the two significant constraints limiting soybean productivity. In order to address these problems, we have transformed soybean cv. Pusa 16 via somatic embryogenesis with salinity induced and apoplastically secreted pathogenesis-related tobacco osmotin (Tbosm) gene using Agrobacterium-mediated genetic transformation. Integration of Tbosm in randomly selected five GUS assay-positive independently transformed soybean plants was confirmed by polymerase chain reaction (PCR) and Southern hybridization. Reverse transcriptase-PCR (RT-PCR) and Western blotting confirmed that the Tbosm was expressed in three of the five transformed soybean plants. Further the Western blotting revealed that the truncated osmotin protein accumulated more in apoplastic fluid. The transformed (T(1)) soybean plants survived up to 200 mM NaCl, whereas non-transformed (NT) plants could withstand till 100 mM and perished at 150 mM NaCl. The biochemical analysis revealed the T(1) soybean plants accumulated higher amount of proline, chlorophyll, APX, CAT, SOD, DHAR, MDHAR, and RWC than NT plants. Leaf gas exchange measurements revealed that T(1) soybean plants maintained higher net photosynthetic rate, CO(2) assimilation, and stomatal conductance than NT plants. The three T(1) soybean plants expressing the osmotin gene also showed resistance against three important fungal pathogens of soybean--Microsphaera diffusa, Septoria glycines and Phakopsora pachyrhizi. The T(1) soybean plants produced 32-35 soybean pods/plant containing 10.3-12.0 g of seeds at 200 mM NaCl, whereas NT plant produced 28.6 soybean pods containing 9.6 g of seeds at 100 mM NaCl. The present investigation clearly shows that expression of Tbosm enhances salinity tolerance and fungal disease resistance in transformed soybean plants.

Direct targets of the tomato-ripening regulator RIN identified by transcriptome and chromatin immunoprecipitation analyses

The physiological and biochemical changes in fruit ripening produce key attributes of fruit quality including color, taste, aroma and texture. These changes are driven by the highly regulated and synchronized activation of a huge number of ripening-associated genes. In tomato (Solanum lycopersicum), a typical climacteric fruit, the MADS-box transcription factor RIN is one of the earliest-acting ripening regulators, required for both ethylene-dependent and ethylene-independent pathways. Although we previously identified several direct RIN targets, many additional targets remain unidentified, likely including key ripening-associated genes. Here, we report the identification of novel RIN targets by transcriptome and chromatin immunoprecipitation (ChIP) analyses. Transcriptome comparisons by microarray of wild-type and rin mutant tomatoes identified 342 positively regulated genes and 473 negatively regulated genes by RIN during ripening. Most of the positively regulated genes contained possible RIN-binding (CArG-box) sequences in their promoters. Subsequently, we selected six genes from the positively regulated genes and a ripening regulator gene, CNR, and assayed their promoters by quantitative ChIP-PCR to examine RIN binding. All of the seven genes, which are involved in cell wall modification, aroma and flavor development, pathogen defense and transcriptional regulation during ripening, are targets of RIN, suggesting that RIN may control multiple diverse ripening processes. In particular, RIN directly regulates the expression of the ripening-associated transcription factors, CNR, TDR4 and a GRAS family gene, providing an important clue to elucidate the complicated transcriptional cascade for fruit ripening.

The superfamily of thaumatin-like proteins: its origin, evolution, and expression towards biological function

Thaumatin-like proteins (TLPs) are the products of a large, highly complex gene family involved in host defence and a wide range of developmental processes in fungi, plants, and animals. Despite their dramatic diversification in organisms, TLPs appear to have originated in early eukaryotes and share a well-defined TLP domain. Nonetheless, determination of the roles of individual members of the TLP superfamily remains largely undone. This review summarizes recent advances made in elucidating the varied TLP activities related to host resistance to pathogens and other physiological processes. Also discussed is the current state of knowledge on the origins and types of TLPs, regulation of gene expression, and potential biotechnological applications for TLPs.

Levels and Stability of Expression of Transgenes

It is well known that in a given cell, at a particular time, only a fraction of the entire genome is expressed. Expression of a gene, nuclear, or organellar starts with the onset of transcription and ends in the synthesis of the functional protein. The regulation of gene expression is a complex process that requires the coordinated activity of different proteins and nucleic acids that ultimately determine whether a gene is transcribed, and if transcribed, whether it results in the production of a protein that develops a phenotype. The same also holds true for transgenic crops, which lie at the very core of insert design.

There are multiple checkpoints at which the expression of a gene can be regulated and controlled. Much of the emphasis of studies related to gene expression has been on regulation of gene transcription, and a number of methods are used to effect the control of gene expression. Controlling transgene expression for a commercially valuable trait is necessary to capture its value. Many gene functions are either lethal or produce severe deformity (resulting in loss of value) if over-expressed. Thus, expression of a transgene at a particular site or in response to a particular elicitor is always desirable.

Two rice phosphate transporters, OsPht1;2 and OsPht1;6, have different functions and kinetic properties in uptake and translocation

Plant phosphate (Pi) transporters mediate the uptake and translocation of this nutrient within plants. A total of 13 sequences in the rice (Oryza sativa) genome can be identified as belonging to the Pi transporter (Pht1) family. Here, we report on the expression patterns, biological properties and the physiological roles of two members of the family: OsPht1;2 (OsPT2) and OsPht1;6 (OsPT6). Expression of both genes increased significantly under Pi deprivation in roots and shoots. By using transgenic rice plants expressing the GUS reporter gene, driven by their promoters, we detected that OsPT2 was localized exclusively in the stele of primary and lateral roots, whereas OsPT6 was expressed in both epidermal and cortical cells of the younger primary and lateral roots. OsPT6, but not OsPT2, was able to complement a yeast Pi uptake mutant in the high-affinity concentration range. Xenopus oocytes injected with OsPT2 mRNA showed increased Pi accumulation and a Pi-elicited depolarization of the cell membrane electrical potential, when supplied with mM external concentrations. Both results show that OsPT2 mediated the uptake of Pi in oocytes. In transgenic rice, the knock-down of either OsPT2 or OsPT6 expression by RNA interference significantly decreased both the uptake and the long-distance transport of Pi from roots to shoots. Taken together, these data suggest OsPT6 plays a broad role in Pi uptake and translocation throughout the plant, whereas OsPT2 is a low-affinity Pi transporter, and functions in translocation of the stored Pi in the plant.

Heat shock factors HsfB1 and HsfB2b are involved in the regulation of Pdf1.2 expression and pathogen resistance in Arabidopsis

In order to assess the functional roles of heat stress-induced class B-heat shock factors in Arabidopsis, we investigated T-DNA knockout mutants of AtHsfB1 and AtHsfB2b. Micorarray analysis of double knockout hsfB1/hsfB2b plants revealed as strong an up-regulation of the basal mRNA-levels of the defensin genes Pdf1.2a/b in mutant plants. The Pdf expression was further enhanced by jasmonic acid treatment or infection with the necrotrophic fungus Alternaria brassicicola. The single mutant hsfB2b and the double mutant hsfB1/B2b were significantly improved in disease resistance after A. brassicicola infection. There was no indication for a direct interaction of Hsf with the promoter of Pdf1.2, which is devoid of perfect HSE consensus Hsf-binding sequences. However, changes in the formation of late HsfA2-dependent HSE binding were detected in hsfB1/B2b plants. This suggests that HsfB1/B2b may interact with class A-Hsf in regulating the shut-off of the heat shock response. The identification of Pdf genes as targets of Hsf-dependent negative regulation is the first evidence for an interconnection of Hsf in the regulation of biotic and abiotic responses.

Identification of a calmodulin-binding NAC protein as a transcriptional repressor in Arabidopsis

Calmodulin (CaM), a ubiquitous calcium-binding protein, regulates diverse cellular functions by modulating the activity of a variety of proteins. However, little is known about how CaM directly regulates transcription. Screening of an Arabidopsis cDNA expression library using horseradish peroxidase-conjugated calmodulin as a probe identified a calmodulin-binding NAC protein (CBNAC). Using gel overlay assays, a Ca2+-dependent CaM-binding domain was identified in the C terminus of this protein. Specific binding of CaM to CaM-binding domain was corroborated by site-directed mutagenesis and a split-ubiquitin assay. Using a PCR-mediated random binding site selection method, we identified a DNA-binding sequence (CBNACBS) for CBNAC, which consisted of a GCTT core sequence flanked on both sides by other frequently repeating sequences (TTGCTTANNNNNNAAG). CBNAC was able to bind to CBNACBS, which resulted in the repression of transcription in Arabidopsis protoplasts. Interestingly, the transcriptional repression mediated by CBNAC was enhanced by CaM. These results suggest that CBNAC may be a CaM-regulated transcriptional repressor in Arabidopsis.

Combinative effects of a bacterial type-III effector and a biocontrol bacterium on rice growth and disease resistance

Expression of HpaG(Xoo), a bacterial type-III effector, in transgenic plants induces disease resistance. Resistance also can be elicited by biocontrol bacteria. In both cases, plant growth is often promoted. Here we address whether biocontrol bacteria and HpaG(Xoo) can act together to provide better results in crop improvement. We studied effects of Pseudomonas cepacia on the rice variety R109 and the hpaG(Xoo)-expressing rice line HER1. Compared to R109, HER1 showed increased growth, grain yield, and defense responses toward diseases and salinity stress. Colonization of roots by P. cepacia caused 20% and 13% increase, in contrast to controls, in root growth of R109 and HER1. Growth of leaves and stems also increased in R109 but that of HER1 was inhibited. When P. cepacia colonization was subsequent to plant inoculation with Rhizoctonia solani, a pathogen that causes sheath blight, the disease was less severe than controls in both R109 and HER1; HER1, nevertheless, was more resistant, suggesting that P. cepacia and HpaG(Xoo) cooperate in inducing disease resistance. Several genes that critically regulate growth and defense behaved differentially in HER1 and R109 while responding to P. cepacia. In R109 leaves, the OsARF1 gene, which regulates plant growth, was expressed in consistence with growth promotion by P. cepacia. Inversely, OsARF1 expression was coincident with inhibition in growth of HER1 leaves. In both plants, the expression of OsEXP1, which encodes an expansin protein involved in plant growth,was concomitant with growth promotion in leaves instead of roots,in response to P. cepacia . We also studied OsMAPK, a gene that encodes a mitogen-activated protein kinase and controls defense responses toward salinity and infection by pathogens in rice. In response to P. cepacia, an early expression of OsMAPK was coincident with R109 resistance to the disease, while HER1 expressed the gene similarly whether P. cepacia was present or not. Evidently, P. cepacia and G(Xoo)-gene mediated resistance may act differently in rice growth and resistance. Whereas combinative effects of P. cepacia and HpaG(Xoo) in disease resistance have a great potential in agricultural use, it is interesting to study mechanisms that underlie interactions involving biocontrol bacteria, type-III effectors and pathogens.

Isolation of a calmodulin-binding transcription factor from rice (Oryza sativa L.)

Calmodulin (CaM) regulates diverse cellular functions by modulating the activities of a variety of enzymes and proteins. However, direct modulation of transcription factors by CaM has been poorly understood. In this study, we isolated a putative transcription factor by screening a rice cDNA expression library by using CaM:horse-radish peroxidase as a probe. This factor, which we have designated OsCBT (Oryza sativa CaM-binding transcription factor), has structural features similar to Arabidopsis AtSRs/AtCAMTAs and encodes a 103-kDa protein because it contains a CG-1 homology DNA-binding domain, three ankyrin repeats, a putative transcriptional activation domain, and five putative CaM-binding motifs. By using a gel overlay assay, gel mobility shift assays, and site-directed mutagenesis, we showed that OsCBT has two different types of functional CaM-binding domains, an IQ motif, and a Ca(2+)-dependent motif. To determine the DNA binding specificity of OsCBT, we employed a random binding site selection method. This analysis showed that OsCBT preferentially binds to the sequence 5'-TWCG(C/T)GTKKKKTKCG-3' (W and K represent A or C and T or G, respectively). OsCBT was able to bind this sequence and activate beta-glucuronidase reporter gene expression driven by a minimal promoter containing tandem repeats of these sequences in Arabidopsis leaf protoplasts. Green fluorescent protein fusions of two putative nuclear localization signals of OsCBT, a bipartite and a SV40 type, were predominantly localized in the nucleus. Most interestingly, the transcriptional activation mediated by OsCBT was inhibited by co-transfection with a CaM gene. Taken together, our results suggest that OsCBT is a transcription activator modulated by CaM.

The ethylene-, jasmonate-, abscisic acid- and NaCl-responsive tomato transcription factor JERF1 modulates expression of GCC box-containing genes and salt tolerance in tobacco

Ethylene responsive factors (ERFs) are important plant-specific transcription factors, some of which have been demonstrated to interact with the ethylene-responsive GCC box and the dehydration-responsive element (DRE); however, data on the roles of ERF proteins in connection with various signaling pathways are limited. In this research, we used the GCC box, an essential cis-acting element responsive to ethylene and methyl jasmonate (MeJA), as bait in a yeast one-hybrid system to isolate transcription factors from tomato (Lycopersicon esculentum Mill.). One of the cDNAs, which was designated Jasmonate and Ethylene Response Factor 1 (JERF1), encodes an ERF protein, containing a conserved ERF DNA-binding motif and functioning as a transcriptional activator in yeast through targeting to the nucleus in onion (Allium cepa L.) epidermal cells. Biochemical analysis revealed that JERF1 bound not only to the GCC box but also to the DRE sequence. Expression of the JERF1 gene in tomato was induced by ethylene, MeJA, abscisic acid (ABA) and salt treatment, indicating that JERF1 might act as a connector among different signal transduction pathways. Further research with transgenic JERF1 tobacco (Nicotiana tabacum L.) plants indicated that overexpressing JERF1 activated expression of GCC box-containing genes such as osmotin, GLA, Prb-1b and CHN50 under normal growth conditions, and subsequently resulted in enhanced tolerance to salt stress, suggesting that JERF1 modulates osmotic tolerance by activation of downstream gene expression through interaction with the GCC box or DRE.

Direct interaction of a divergent CaM isoform and the transcription factor, MYB2, enhances salt tolerance in arabidopsis

Calmodulin (CaM), a ubiquitous calcium-binding protein, regulates diverse cellular functions by modulating the activity of a variety of enzymes and proteins. Plants express numerous CaM isoforms that exhibit differential activation and/or inhibition of CaM-dependent enzymes in vitro. However, the specific biological functions of plant CaM are not well known. In this study, we isolated a cDNA encoding a CaM binding transcription factor, MYB2, that regulates the expression of salt- and dehydration-responsive genes in Arabidopsis. This was achieved using a salt-inducible CaM isoform (GmCaM4) as a probe from a salt-treated Arabidopsis expression library. Using domain mapping, we identified a Ca2+-dependent CaM binding domain in MYB2. The specific binding of CaM to CaM binding domain was confirmed by site-directed mutagenesis, a gel mobility shift assay, split ubiquitin assay, and a competition assay using a Ca2+/CaM-dependent enzyme. Interestingly, the specific CaM isoform GmCaM4 enhances the DNA binding activity of AtMYB2, whereas this was inhibited by a closely related CaM isoform (GmCaM1). Overexpression of Gm-CaM4 in Arabidopsis up-regulates the transcription rate of AtMYB2-regulated genes, including the proline-synthesizing enzyme P5CS1 (Delta1-pyrroline-5-carboxylate synthetase-1), which confers salt tolerance by facilitating proline accumulation. Therefore, we suggest that a specific CaM isoform mediates salt-induced Ca2+ signaling through the activation of an MYB transcriptional activator, thereby resulting in salt tolerance in plants.

Overexpression of the AP2/EREBP transcription factor OPBP1 enhances disease resistance and salt tolerance in tobacco

Osmotin promoter binding protein 1 (OPBP1), an AP2/EREBP-like transcription factor of tobacco (Nicotiana tabacum), was isolated using a yeast one-hybrid system. RNA gel blot analysis indicated that expression of the OPBP1 gene was induced by elicitor cryptogein, NaCl, ethephon, methyl jasmonate, as well as cycloheximide. Transient expression analysis using an OPBP1-eGFP fusion gene in onion epidermal cells revealed that the OPBP1 protein was targeted to the nuclear. Further, electrophoretic mobility shift assays demonstrated that the recombinant OPBP1 protein could bind to an oligonucleotide containing the GCC-box cis element. Transgenic tobacco plants with an over expression of the OPBP1 gene accumulated high levels of PR-1a and PR-5d genes and exhibited enhanced resistance to infection by Pseudomonas syringae pv tabaci and Phytophthora parasitica var nicotianae pathogens. They also exhibited increased tolerance to salt stress. These results suggest that OPBP1 might be a transcriptional regulator capable of regulating expression in sets of stress-related genes.

A role for the GCC-box in jasmonate-mediated activation of the PDF1.2 gene of Arabidopsis

The PDF1.2 gene of Arabidopsis encoding a plant defensin is commonly used as a marker for characterization of the jasmonate-dependent defense responses. Here, using PDF1.2 promoter-deletion lines linked to the beta-glucoronidase-reporter gene, we examined putative promoter elements associated with jasmonate-responsive expression of this gene. Using stably transformed plants, we first characterized the extended promoter region that positively regulates basal expression from the PDF1.2 promoter. Second, using promoter deletion constructs including one from which the GCC-box region was deleted, we observed a substantially lower response to jasmonate than lines carrying this motif. In addition, point mutations introduced into the core GCC-box sequence substantially reduced jasmonate responsiveness, whereas addition of a 20-nucleotide-long promoter element carrying the core GCC-box and flanking nucleotides provided jasmonate responsiveness to a 35S minimal promoter. Taken together, these results indicated that the GCC-box plays a key role in conferring jasmonate responsiveness to the PDF1.2 promoter. However, deletion or specific mutations introduced into the core GCC-box did not completely abolish the jasmonate responsiveness of the promoter, suggesting that the other promoter elements lying downstream from the GCC-box region may also contribute to jasmonate responsiveness. In other experiments, we identified a jasmonate- and pathogen-responsive ethylene response factor transcription factor, AtERF2, which when overexpressed in transgenic Arabidopsis plants activated transcription from the PDF1.2, Thi2.1, and PR4 (basic chitinase) genes, all of which contain a GCC-box sequence in their promoters. Our results suggest that in addition to their roles in regulating ethylene-mediated gene expression, ethylene response factors also appear to play important roles in regulating jasmonate-responsive gene expression, possibly via interaction with the GCC-box.

Ectopic expression of Tsi1 in transgenic hot pepper plants enhances host resistance to viral, bacterial, and oomycete pathogens

In many plants, including hot pepper plants, productivity is greatly affected by pathogen attack. We reported previously that tobacco stress-induced gene 1 (Tsi1) may play an important role in regulating stress responsive genes and pathogenesis-related (PR) genes. In this study, we demonstrated that overexpression of Tsi1 gene in transgenic hot pepper plants induced constitutive expression of several PR genes in the absence of stress or pathogen treatment. The transgenic hot pepper plants expressing Tsi1 exhibited resistance to Pepper mild mottle virus (PMMV) and Cucumber mosaic virus (CMV). Furthermore, these transgenic plants showed increased resistance to a bacterial pathogen, Xanthomonas campestris pv. vesicatoria and also an oomycete pathogen, Phytophthora capsici. These results suggested that ectopic expression of Tsi1 in transgenic hot pepper plants enhanced the resistance of the plants to various pathogens, including viruses, bacteria, and oomycete. These results suggest that using transcriptional regulatory protein genes may contribute to developing broad-spectrum resistance in crop plants.

Regulated expression of Arabidopsis phosphate transporters

Phosphorus deficiency is one of the major abiotic stresses affecting plant growth. Plants respond to the persistent deficiency of phosphate (Pi) by coordinating the expression of genes involved in alleviation of the stress. The high-affinity Pi transporters are among the major molecular determinants that are activated during Pi stress. In this study, using three reporter genes (green fluorescent protein, luciferase, and beta-glucuronidase) regulated by two Pi transporter promoters, we have carried out an extensive analysis of transcriptional and spatial regulation of gene expression. Activation of the genes was rapid, repressible, and specific in response to changes in Pi availability. The phytohormones auxin and cytokinin suppressed the expression of the reporter gene driven by the AtPT1 promoter, and that of the native gene, suggesting that hormones may be involved in regulation of some component(s) of Pi starvation response pathway. These studies also provide molecular evidence for a potential role of high-affinity Pi transporters in mobilizing Pi into reproductive organs. The results suggest that members of the Pi transporter family may have similar but nonredundant functions in plants.

Cloning and DNA-binding properties of a tobacco Ethylene-Insensitive3 (EIN3) homolog

Ethylene-Insensitive3 (EIN3) is a transcription factor that works in the ethylene signaling pathway in Arabidopsis. We isolated a tobacco cDNA encoding an EIN3 homolog as a sequence-specific DNA-binding protein. The encoded protein TEIL (tobaccoEIN3-like) shares 60% identity in amino acid sequence with EIN3. The DNA-binding domain was localized in the N-terminal half, which shows 92% identity in amino acid sequence with the corresponding region of EIN3, suggesting a conserved function in DNA-binding specificity. TEIL was indeed functionally similar to EIN3 because, like EIN3-overexpressing plants, transgenic Arabidopsis seedlings overexpressing TEIL cDNA exhibited constitutive triple response phenotypes. Random binding site selection analysis revealed that the consensus binding sequence for TEIL is AYGWAYCT, where Y and W represent A or C and A or T, respectively. A reporter plasmid containing the TEIL binding sites showed a 7- to 10-fold higher activation relative to that containing a mutated TEIL-binding sequence in tobacco protoplasts. A further 2- to 3-fold increase in activation was observed when a plasmid for TEIL overproduction was co-transfected, indicating that TEIL is a transcriptional activator. Moreover, nuclear extracts from ethylene-treated leaves showed an increase in DNA-binding activity specific to the TEIL-binding sequence, despite the level of the transcripts being unchanged. These observations suggest that TEIL functions as a transcription activator with a relatively redundant DNA-binding specificity, and its function may be regulated at least in part by modulation of the DNA-binding activity through ethylene signaling.

Receptors and signalling components of plant hormones

Recent advances in understanding plant hormonal signalling has resulted in the identification of a variety of signalling components including receptor kinases with homology to the bacterial two component system as well as serine/threonine kinases and protein phosphatases. In addition, the existence of MAP kinase pathways in plants indicates a similar role of these signalling cascades in the relay of exogenous signals into the nucleus as has been disclosed in animal cells. The emerging signalling pathways of the plant hormone abscisic acid and ethylene are presented.

A nitrilase-like protein interacts with GCC box DNA-binding proteins involved in ethylene and defense responses

Ethylene-responsive element-binding proteins (EREBPs) of tobacco (Nicotiana tabacum L.) bind to the GCC box of many pathogenesis-related (PR) gene promoters, including osmotin (PR-5). The two GCC boxes on the osmotin promoter are known to be required, but not sufficient, for maximal ethylene responsiveness. EREBPs participate in the signal transduction pathway leading from exogenous ethylene application and pathogen infection to PR gene induction. In this study EREBP3 was used as bait in a yeast two-hybrid interaction trap with a tobacco cDNA library as prey to isolate signal transduction pathway intermediates that interact with EREBPs. One of the strongest interactors was found to encode a nitrilase-like protein (NLP). Nitrilase is an enzyme involved in auxin biosynthesis. NLP interacted with other EREBP family members, namely tobacco EREBP2 and tomato (Lycopersicon esculentum L.) Pti4/5/6. The EREBP2-EREBP3 interaction with NLP required part of the DNA-binding domain. The specificity of interaction was further confirmed by protein-binding studies in solution. We propose that the EREBP-NLP interaction serves to regulate PR gene expression by sequestration of EREBPs in the cytoplasm.

ABA signal transduction

Recent advances in the study of abscisic acid signal transduction include the identification of cyclic ADP-ribose as a central mediator of abscisic acid responses. The characterisation of type 2C protein phosphatases, ABI1 and ABI2, implicates negative control and redundant action on the signal pathway of this hormone. In addition, abscisic acid-mediated inhibition of gibberellin-stimulated responses seems to depend on the activation of a phospholipase D during induction of alpha-amylase in barley aleurone cells as well as on a putative acetyltransferase involved in elongation growth.